The present invention relates to the art of magnetic resonance spectroscopy. More particularly, the present invention relates to coils for receiving electromagnetic signals from resonating nuclei. Although the present invention finds application in the field of medical imaging, it is to be appreciated that the invention also finds utility in other magnetic resonance applications, such as well logging, chemical analysis, and the like.
Heretofore, there have been two primary categories of receiving coils for magnetic resonance imaging and spectroscopy of selected body areas of the patient. First, a standard sized whole body coil or body portion receiving coil was dimensioned to be disposed around the portion of the patient to be imaged. Due to the standard sizing, a significant void or empty region was defined between the coil and the portion of the patient to be imaged. As the imaged portion of the patient became a smaller fraction of the coil volume, the signal-to-noise ratio was decreased, degrading the image quality. Further, the standard sized coils included no means to eliminate or reduce aliasing in two or three dimensional Fourier transform sequences. The other type of receiving coil was formed by wrapping wire or other conductors on flat, rigid sheets of plastic or nylon. The flat coils were constructed in a variety of sizes to facilitate being positioned adjacent an area of the patient to be imaged. However, their planar nature permitted only limited, partial contact with the patient.
The rigid, standard sized body and body portion coils and the rigid, planar coils did not permit optimization of image quality. Rather, the lack of conformity with the patient failed to optimize the filling factor decreasing the signal-to-noise ratio. These rigid coils received resonance signals from over a significantly larger area than the region of interest. This sensitivity to extraneous information degraded spatial resolution and increased aliasing in two and three dimensional Fourier transform methodology. Improvement in the homogeneity of receiver sensitivity across the imaged space sacrificed the quality or Q factor of the coil, particularly in coils having resonance frequencies above 20 MHz. Moreover, the rigid coils were difficult to apply to the patient, uncomfortable, and created a need for a large range inventory of coil sizes.
Other problems have been encountered in transferring the signal received by the high impedance coil over low impedance transmission lines to a remote, out of the image region preamplifier. To minimize signal loss and noise, the length of the low impedance transmission line was minimized. Although short unmatched transmission lines functioned acceptably at low frequencies, the Q factor of the coil degraded rapidly with increasing frequency and cable length.
Matching the transmission line length to the wave length at the operating frequency resulted in excessive length at low and mid-field strengths and lines that were too short at high fields. Because the coil impedance greatly exceeded the transmission line impedance, high cable losses attributable to standing waves on the cable were experienced. Moreover, non-zero cable dielectric and conductor losses damped the surface coil.
Attempting to match the impedance of the transmission line, whether balanced or unbalanced, has been unsuccessful. The normal variations in patient loading caused a corresponding impedance mismatch and resultant power transfer loss. At mid and high magnetic field strengths the patient loading mismatch and transmission loss were magnified.
The present invention provides a new and improved coil and coil signal transmission system which overcomes the above referenced problems and others.